Surface ordering in thin films of liquid-crystalline polymers containing fluorinated and protonated segments: Neutron-reflectometry study

Surface-induced layering of fluorinated and protonated segments in thin films of a main-chain liquid-crystalline polymer, consisting of α-methylstilbene, bridged by a fluorinated group was revealed by neutron reflectometry. The layering was driven by the difference in surface energy of the fluorinated and protonated segments and by the inherent ordering of the polymer. The lower-surface-energy fluorinated segments segregated to the air surface, and the protonated segments segregated to the SiO_x layer at the Si substrate. The ordering induced by the interface decayed into the film with a characteristic decay length of about 100 Å. The surface-induced periodicity ranged from 15 to 20 Å, which is approximately equal to the molecular dimension of the repeating unit on the polymer backbone. The magnitude of segregation increased upon annealing in the liquid-crystalline temperature range. The segregation was retained upon annealing above the bulk order-disorder transition temperature.